1. Field of the Invention
This invention relates to the field of optical recording and playback. More particularly, it relates to improvements in apparatus for controlling the output power of a laser diode used in an optical head for recording/playing back (writing/reading) information on optical recording media.
2. The Prior Art
Laser diodes are commonly used in optical write/read heads to record and playback encoded information on optical discs and the like. In FIG. 1, for example, a conventional write/read head is shown as comprising a laser diode LD which produces a beam B of plane-polarized radiation. During writing of information on an optical disc D, the beam intensity is modulated by a data signal with laser power controlled through a suitable power circuit PC. The output of the laser diode is collimated by a lens L1 which directs the collimated beam B' through the combination of a polarizing beam splitter PBS and quarter-wave plate QWP. The beam splitter comprises a polarizing filter F which is oriented to transmit radiation in the plane in which beam B' is polarized, and the quarter-wave plate operates to circularly polarize the plane-polarized beam incident thereon. The circularly polarized beam from the quarter-wave plate is brought to focus on the recording layer of the optical disc by condenser lens L2. Upon being reflected by the optical disc, the direction or "handedness" in which the incident beam is circularly polarized is reversed. Upon passing again through the quarter-wave plate, the circularly polarized reflected beam becomes plane-polarized, this time in the direction perpendicular to the plane of polarization of the laser beam B. The polarizing filter of the beam splitter now acts to reflect the beam reflected from the disc toward a photodetector PD, and a condensing lens L3 brings such beam to focus on the photodetector. Both the front and rear surfaces of the quarter-wave plate, as well as various other surfaces in the optical path, are commonly provided with an anti-reflection coating AR to minimize optical transmission losses and stray light reflections. The above combination of the quarter-wave plate and polarizing beam splitter is commonly used to isolate the laser cavity from light reflected by the disc, thereby stabilizing, to some extent, the laser output power.
Generally speaking, laser diodes are well suited for use in optical recording applications for reasons which include size, power, phase coherency and monochromaticity. A drawback of such devices, however, is that the output power, as reflected by the beam intensity, tends to fluctuate, particularly with temperature changes and optical feedback. Such power fluctuations can give rise to the formation of non-uniform pits in the recording process, as well as errors in the playback signal. As a result, it has been common practice to provide some form of optical feedback in order to automatically and continually stabilize the laser's output power. For example, it has been proposed to mount a photodetector within the laser diode housing or "can" for the purpose of monitoring the emitted light level. The photodetector output is then fed back to a control circuit which, in response to the signal, drives the laser in such a manner as to maintain its radiated output within certain limits. While this approach is theoretically useful for controlling laser power, it is disadvantageous from a variety of standpoints, including performance. Stray light in the can, as produced, for example, by multiple reflections of the write/read beam before it exits the can, or by other non-selected beams produced by the laser diode, can produce an error in the photodetector signal, thereby rendering such signal non-indicative of the power in the beam of interest.
Rather than monitoring the laser beam intensity before the beam emerges from the laser diode's housing, several schemes have been proposed for directing a small portion of the laser diodes' output beam to a photodetector located outside the laser diode's housing. One such scheme is disclosed in Japanese Published Application No. 4-109437, published on Apr. 10, 1992. As shown in FIG. 2 wherein like reference characters denote like parts in the FIG. 1 apparatus, a laser beam B emitted by a laser diode is collimated and passed through a diffraction grating G to produce three discrete and closely spaced parallel beams (shown as one). The central beam of the three beams is used to write or read information on a magneto-optical disc D', and the outboard beams are used for tracking purposes. The three beams are directed through a non-polarizing beam splitter BS comprising a dielectric film F' which is arranged at 45 degrees relative to the optical path and adapted to transmits the major portion of the three beams to the recording disc. Upon being reflected by the disc, the three beams are reflected by the dielectric film F' towards a photodetector array PD1 which is used to derive focus, tracking and recorded signal information. The dielectric film in the beam splitter also functions to reflect a minority portion of each beam to a lateral surface S of the beam splitter, such surface being inclined at a small angle .alpha. from normal to the direction of the incident beam path. Preferably, surface S has no anti-reflection coating, whereby about 5% of the incident light is reflected towards a second photodetector array PD2 which is laterally displaced from photodetector PD1 and functions to sense the laser beam power and provide a feedback control signal to a laser control circuit CC which controls the laser power. Presumably, the non-reflected portion of the beam striking surface S is transmitted, absorbed or otherwise lost.
While laser power-monitoring systems of the above type may prove useful in maintaining the output of a laser diode relatively constant, they do so at the expense of a relatively costly optical element, namely, the beam splitter BS which is relatively unusual both in terms of shape and function. It would be highly desirable to be able to monitor laser power without the cost of any additional optical elements other than those required for writing and reading information on the recording media.